Sorry for not posting 'till now... Because of a major hard drive crash I've been busy recovering data... ~.~
First of all thanks to you Ari! It's good to see a game designer really caring about his creation and taking account into the suggestion of gamers for the benefit of all
Okay, I still have to review the suggested tweaks to the mystery-casting system, however there are still some moderate flaws to the mechanics which IMHO need correction.
I've always been a friend of using existing rules first and adding new mechanics if absolutely necessary, only. Every new rule further complicates a game and makes a DM's live (and to a lesser degree a Player's live, too) harder, especially when introducing new claasses with new / different mechanics. Let's face it, in actual game play, players tend to remember all bonuses their characters get, but all too easy forget some penalties left and right...
That said take a look at the
Mysteries and Paths section (TM, p. 138):
Some of the benefits / penalty sounded quite familiar, especially the penalties for spell/mystery interaction and the penalty to spellcraft checks.
Following the approach already given in the
Spell Thematics and
Tenacious Magic feats from
Players Guide to Faerûn (p. 44-45), I'd suggest a 'one-way' penalty only.
Further I don't know, if the introduction of Metashadow feats is
really necessary. I haven't had the time to check every single Metamagic feat, however I guess restricting mystery-casting classes to Metamagic feats that do not use a higher level spell slot (because mystery-users don't have any spell-slots in the first place) would do just as fine. New [Metamagic] feats for mystery-casting classes only, could have the prequisite "mystery-using class". In my experience sticking to the rules already known to all is always better than introducing something new and I really don't know if the game needs just another kind of Feats, especially as I fear we won't see much (if any) support of shadow magic in future supplements. While there would be lots of new options for other classes, keeping them flexible, the shadowcaster would remain static and become more and more unattractive to players over time.
Restriction of feats like Ability Focus and Empower Spell-like Ability is another matter I'd not see as a necessity. Do you really think access to those Feats would be that unbalancing?

Haven't thought about it too intensely, but spontanously I'd suggest a mystery affected by e.g. Empower Spell-like Ability would become a
real spell-like ability and stop being a mystery. The consequences? Lower level mysteries won't become supernatural abilities and Metashadow feats cannot be applied to the ex-mystery anymore. At least at first glance it sounds like a fair trade-off to me, doesn't it?
Prestige class restriction (TM p. 117) is a really tricky matter. On the one hand it's quite hard to restrict shadowcasters to that few options, on the other hand I understand the reasons why. Hm... Maybe a simple official list (web enhancement?) of published prestige classes a shadowcaster would qualify for would do?
Btw. I'd
really like to see a ...uhm... 'more detailed' version of the
The Shadow Weave of Toril side-box given on page 110 of the TM. As it is, it's rather a bunch of useless information. Is a shadowcaster automatically considered a Shadow Weave user (e.g. gaining the
Shadow Weave Magic feat as a bonus feat?) ? Should a shadowcaster get access to the set of Shadow Weave Feats (two of them being Metamagic feats)? Shouldn't shadowcasters qualify for the Shadow Adept prestige class?
The suggestion about the Mysteries and Paths section above would solve some of the problems, as you actually could rule shadowcasters being shadow weave users, gaining the Shadow Weave Magic and Tenacious Magic feats for free.
Well, I'll try to take a deeper look at the suggested changes to the mystery-casting mechanics later that week.